Thermally-operated switching devices used as relays, signal flashers, or the like, are well known. Such devices can be made snap-acting by providing an element which moves over center between two stable positions with heating or cooling of a thermal element. Bimetal relays in which a heater element causes heating of an active bimetal unit are well known. Such devices are inherently relatively slow acting because of large thermal masses involved. Snap-acting springs with separate tension bands or ribbons that are thermally heated and cooled to bend the spring have been proposed for switching devices, such as flashers. See U.S. Pats. Nos. 3,174,012 and 3,305,654, for example. The band can be made of very thin metal to provide a low thermal mass that can be more quickly heated and cooled than a bimetal element. Such devices have used the band as an electrical conductor to provide resistance heating. Also separate heaters spaced from the band have been used, or the band is wrapped with a resistance heater wire that is electrically insulated from the band. In such known devices the band does not make good thermal contact with the spring when the device is activated. Thus the rate of cooling by transfer of heat away from the band is relatively poor. This slows the rate at which the device can be cycled. Where the heater is separate from the band, the heat transfer from the heater to the band is further restricted.